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ABSTRACT: Intracerebroventricular (ICV) injections of prolactin (PRL) exert potent antigonadal and antigonadotropic effects in ring doves (Streptopelia risoria) at doses that are insufficient to stimulate prolactin-dependent crop growth. To explore the physiological basis of these effects, we tested the ability of ICV-injected PRL to influence pituitary responsiveness to chicken gonadotropin-releasing hormone-I (cGnRH-I) and to alter GnRH content and concentration in the preoptic area (POA) and median eminence (ME). cGnRH-I-induced changes in plasma LH were monitored by radioimmunoassay (RIA) in photostimulated male doves after they received five daily ICV injections of ovine PRL (1 microg/2 microl) or saline vehicle. Although PRL treatment reduced basal plasma LH levels and testes weight, it did not reduce the amount or alter the pattern of LH released in response to a bolus injection of cGnRH-I. This suggests that ICV PRL does not suppress LH by reducing pituitary responsiveness to GnRH. In two subsequent studies, GnRH content (ng/region) and concentration (pg/microg protein) in the POA and ME were measured in male doves by RIA and by competitive enzyme immunoassay after 5 days of ICV PRL or vehicle treatment. Although ICV PRL reduced plasma LH levels in both studies, no significant PRL-induced alterations in GnRH content or concentration were apparent. In a final study, PRL-treated female doves had lower plasma LH levels than vehicle-treated control females at 12 and 24 h after a single ICV injection. GnRH content of the POA was also lower in PRL-treated females than in controls at 24 h. However, the two treatment groups did not differ in POA or ME GnRH content at earlier postinjection sampling intervals. Analysis of GnRH concentration data revealed no treatment group differences in either region at any sampling interval (1, 6, 12, or 24 h post-PRL injection). Collectively, these results are consistent with the idea that ICV-injected PRL acts at the level of the CNS to inhibit the reproductive system. However, the nature of the alterations involved remains to be clarified. Plausible hypotheses are (1) that ICV PRL suppresses the gonadal axis by influencing the activity of GnRH neurons at brain sites other than the POA or ME or (2) that PRL alters the synthesis, storage, degradation, and/or release of GnRH in the POA or ME, but the dynamic changes involved are not reflected in integrated, steady-state measures such as peptide content or concentration in tissue.
General and Comparative Endocrinology 05/1999; 114(1):97-107. · 3.27 Impact Factor
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ABSTRACT: Wilson's Phalarope (Phalaropus tricolor) is a sex role-reversed species in which incubation of eggs and care of young is performed exclusively by the male. Plasma levels of prolactin (PRL), the hormone most associated with parental care in birds, are higher in incubating males than in nonincubating males or females. Conversely, plasma testosterone levels are reduced in males during incubation. In an attempt to characterize the physiological basis of this unusual parental care system we used quantitative film autoradiography and densitometry to measure the specific binding in vitro of 125I-ovine PRL to 12 brain regions in females, nonincubating males, and incubating males during the normal breeding season. We also measured hypothalamic chicken gonadotropin-releasing hormone I (cGnRH-I) in three brain areas in these same birds, as well as plasma levels of PRL and testosterone. Analysis revealed that cGnRH-I concentrations in the preoptic area and plasma testosterone levels were significantly lower in incubating males than in nonincubating males. Specific binding of 125I-ovine PRL was detected in choroid plexus and in several diencephalic brain regions of both sexes, with highest binding activity recorded in the dorsolateral thalamus, medial habenula, nucleus subrotundus, and preoptic area. When adjustments were made for the large number of comparisons performed, specific binding did not vary significantly by sex or breeding stage in any single brain region. However, average specific binding values in nonincubating males exceeded those of incubating males in 9 of the 11 PRL-sensitive regions examined. Increased occupancy of the receptor by endogenous PRL during incubation could have contributed to this result, since plasma PRL levels were elevated in incubating males. In addition, PRL binding activity in several of these brain regions tended to correlate negatively with plasma PRL. The two exceptions to this general pattern were the preoptic area and the lateral septum, where mean specific binding was 14-15% higher in incubating males than in nonincubating males. This raises the interesting possibility that PRL sensitivity is up-regulated during incubation in some regions of the male phalarope brain, such as the preoptic area and lateral septum, that have been implicated in PRL-modulated changes in behavior and reproductive activity during this breeding stage.
General and Comparative Endocrinology 02/1998; 109(1):119-32. · 3.27 Impact Factor
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ABSTRACT: Intracerebroventricular (ICV) injections of prolactin (PRL) exert potent antigonadal and antigonadotropic effects in ring doves (Streptopeliarisoria) at doses that are insufficient to stimulate prolactin-dependent crop growth. To explore the physiological basis of these effects, we tested the ability of ICV-injected PRL to influence pituitary responsiveness to chicken gonadotropin-releasing hormone-I (cGnRH-I) and to alter GnRH content and concentration in the preoptic area (POA) and median eminence (ME). cGnRH-I-induced changes in plasma LH were monitored by radioimmunoassay (RIA) in photostimulated male doves after they received five daily ICV injections of ovine PRL (1 μg/2 μl) or saline vehicle. Although PRL treatment reduced basal plasma LH levels and testes weight, it did not reduce the amount or alter the pattern of LH released in response to a bolus injection of cGnRH-I. This suggests that ICV PRL does not suppress LH by reducing pituitary responsiveness to GnRH. In two subsequent studies, GnRH content (ng/region) and concentration (pg/μg protein) in the POA and ME were measured in male doves by RIA and by competitive enzyme immunoassay after 5 days of ICV PRL or vehicle treatment. Although ICV PRL reduced plasma LH levels in both studies, no significant PRL-induced alterations in GnRH content or concentration were apparent. In a final study, PRL-treated female doves had lower plasma LH levels than vehicle-treated control females at 12 and 24 h after a single ICV injection. GnRH content of the POA was also lower in PRL-treated females than in controls at 24 h. However, the two treatment groups did not differ in POA or ME GnRH content at earlier postinjection sampling intervals. Analysis of GnRH concentration data revealed no treatment group differences in either region at any sampling interval (1, 6, 12, or 24 h post-PRL injection). Collectively, these results are consistent with the idea that ICV-injected PRL acts at the level of the CNS to inhibit the reproductive system. However, the nature of the alterations involved remains to be clarified. Plausible hypotheses are (1) that ICV PRL suppresses the gonadal axis by influencing the activity of GnRH neurons at brain sites other than the POA or ME or (2) that PRL alters the synthesis, storage, degradation, and/or release of GnRH in the POA or ME, but the dynamic changes involved are not reflected in integrated, steady-state measures such as peptide content or concentration in tissue.
General and Comparative Endocrinology.
